Probably one of the biggest mistakes singers make is that they don’t open up their mouths wide enough. Not every song in the world is going to be right for your voice. Aside from vocal range, there is style as well. You’ll never hear Judy Collins trying to belt out something like “Born To Run”. You have to find songs that fit YOUR voice. This is very easy to do. Pick out something and try singing it. If it doesn’t feel comfortable at all, it’s probably not for you.